If you lose something and you've looked everywhere else, check behind the toilet. That's what your uncle always said. Probably because he had a very specific kind of mental illness that caused him to compulsively steal things from people and hide them behind toilets.
Anyway, it looks like your uncle has been here, because you find a can of drain dissolver behind the toilet.
|You acquire an item: drain dissolver|
Occurs at The Haunted Bathroom.